I have succesfully used the Wiznet 5200 chip with a Due with a clock divider setting of 2, effectively running the SPI at 42MHz. The datasheet for the W5200 says it can go upto 80MHz on the SPI bus. Using 1 as a clock divider would be outside of the spec, giving a rate of 84MHz. I originally tried it at a slow speed and gradually built up to 42MHz.

At this speed I am able to have 8 different browser connections to the Due running as a simple web server all repeatedly requesting the sample page.

I see there was an alternative W5200 library for the Due using DMA on the SPI but with the faster clock speed I felt this wasnt needed for my application and performance has been great, compared to a 328 and W5100 based board.